1. Field of the Invention
The invention relates to manufacturing and testing processes and, more particularly, to improved apparatus for carrying of circuit boards.
2. Background of the Invention
Printed circuit boards carrying electronic and related components are typically produced in a fashion where per-unit cost is a factor to be minimized. Circuit boards may be produced in a high volume manufacturing where any incremental advantage in processing of the circuit boards can effect substantial benefits for the manufacturer. In a typical processing, a bare circuit board may be pretested for continuity, stuffed with components and soldered, cleaned, functionally tested and visually inspected, burned-in according to various criteria, tested, assembled into an assembly, retested, etc. By improving any of the individual processes, a large benefit may result, such as higher yields, lower rework, etc. One such manufacturing advantage may be realized by considering simple quality control logistics and the effects of implementing known testing techniques. Such manufacturing and testing may include handling circuit boards by use of a circuit board carrier.
Many quality control and like methods used in manufacturing are adapted for processing and testing of assembled circuit boards. For example, a failure occurring when a component is first operated may be referred-to as an “infant mortality” that happens during a “burn-in” period. By way of over-simplified example, say a component of a circuit board has a 80% chance of meeting required specifications after an initial operation period. And, in this example, if there are three ‘80% good’ components installed in a circuit board, the likelihood of obtaining a working circuit board becomes (0.80×0.80×0.80)51.2%. If the same three components are each ‘99% good,’ the likelihood of obtaining a working circuit board having the three components becomes (0.99×0.99×0.99)97%. A general rule of thumb is that a cost of culling out failed components increases ten-fold for each successive manufacturing process step. Therefore, it makes sense to efficiently test each component to assure the component is near 100% good, before installing the component in the circuit board. The same principle applies to assembled circuit boards as components of a larger system. Performing testing and various cycling on individual circuit boards is necessary to identify rejects. Such processing should be efficient for reducing manufacturing costs while still optimizing quality. For example, by utilizing a circuit board carrier, various fixturing and handling problems may be simplified. Providing thorough testing of individual circuit boards, before they are installed in a larger piece of equipment, assures higher yield for a final assembly line.
When a circuit board has been stuffed, assembled, soldered, cleaned, tested, visually inspected, etc., the assembled circuit board may be calibrated and functionally tested, for example, by placing the unit under test into a simulation or functional test apparatus. At a convenient point in the manufacturing/testing process, the circuit boards may be loaded into a circuit board carrier. The circuit boards that pass inspection may be loaded via the carrier into an environmental chamber where they are each connected to a load, powered-up, and subjected to increased and/or reduced temperature, humidity, vibration, and other stimuli in a ‘shock cycling’ that accelerates the process of infant mortality. Such cycling may include monitoring certain electrical operating characteristics while the circuit board is undergoing extreme changes in temperature or other environmental parameter. A range for temperature or other cycling parameters typically depends on an intended use for an end product, e.g., consumer goods, military products, etc. For example, consumer type electrical products may have environmental or reliability requirements determined by a regulating agency such as UL, CSA, NEMA, etc. By comparison, a military product having a circuit board may be required to withstand a more intensive burn-in and testing phase, for example by subjecting the circuit board to more extreme temperature shock cycles, vibration, etc. Many potential defects are heat related or a result of a mechanical defect such as a loose wire, and such failures are more likely to occur or manifest themselves by use of such burn-in test cycling, whereupon the defective circuit boards are culled out and reworked or scrapped. In addition, since circuit components are required to interact within a circuit board, the combined functionality of the circuit as a whole is also stressed by the environmental cycling. Circuit boards that survive the environmental testing are more likely to perform successfully in the end product for years to come.
Due to the complexity and number of components in most circuit boards, it is usually required to perform thorough testing, wherein carriers are used for handling and electrical hookups. It is typically highly advantageous to increase the circuit board yield percentage even slightly.
Various kinds of testing may be performed on circuit boards by attaching loads and by doing ‘live’ functional testing either separately or in combination with the environmental cycling. Carriers typically support the circuit boards on edge in a parallel spaced-apart relation and may include electrical connectors for engaging the circuit boards. A carrier may have electrical connectors for electrically connecting the carrier, and its individual circuit boards, to an external device including, for example, a power source, current meter, fuse, signal generator, logic circuit, data collection device, etc. While such carriers generally facilitate efficient manufacturing, they are not adaptable for being easily customized.